docs: def background_mesh_ma(self):
          """
          The background 2D (masked) array mesh prior to any
          interpolation.  The array is masked only if meshes were
          excluded.
          """
  
          if len(self._bkg1d) == self.nboxes:
              return self.background_mesh
          else:
              return self._make_2d_array(self._bkg1d)